We investigated a natural mechanism of resistance to retroviruses that occurs in mice and other species that could provide insights relevant to HIV infection in man. Mice that inherit a gene (designated Fv4) related to a retroviral envelope are resistant to infection with retroviruses that use a particular (ecotropic) virus receptor. It is believed that the resistance is due to blockage of the receptor by endogenously produced envelope (Env) protein, although immune mechanisms could also play a role. We found that transgenic mice that inherit the Fv4 envelope gene do not get immunosuppressed when infected with a murine retrovirus (Friend virus). Mice that express low levels of the Fv4 gene are initially infected by Friend virus but recover from viremia and develop antibodies to viral antigens, whereas non-transgenic mice become chronically infected and do not mount an immune response. Transgenic mice have an expanded pool of CTL precursors specific for viral envelope antigens, including a transgene envelope peptide, and these precursors can be activated in vitro, whereas no such precursors were detected in non-transgenic mice. We do not know if these precursors play a role in the resistance in vivo. We also studied a model tissue culture system for HIV in which HeLa cells expressing the HIV receptor CD4 were stably transfected with a vector encoding HIV env and vpu genes. Dr. Klaus Strebel and others in LMM have previously shown that the HIV env and vpu genes lead to accelerated degradation of CD4. We found that our stable cell lines were composed of mixed populations of cells that had either a small amount of cell surface CD4 and a large amount of intracellular Vpu, or a large amount of cell surface CD4 and a small amount of intracellular Vpu. Treating our cells with brefeldin A led to rapid degradation of intracellular CD4 specifically in the low surface CD4 subpopulation, consistent with the findings of Strebel et al. In contrast, cyclohexamide led to stabilization of intracellular CD4 in these cells, implying that a labile factor may be involved in the degradation pathway. Since rapid degradation of cellular proteins is often mediated by the proteasome pathway, we treated our cells with proteasome inhibitors and found that CD4 degradation was inhibited, suggesting that the proteasome may be involved in Vpu-mediated rapid degradation of CD4. We also worked on a novel "primitive virus" system using a Semliki Forest virus (SFV) vector to express murine retroviral Env protein. One SFV vector encodes SFV capsid and a truncated form of - Moloney MuLV Env that is capable of fusing cells that express the ecotropic MuLV receptor. A separate SFV vector encodes SFV proteins that replicate both vector RNAs. When we introduced these RNAs into BHK cells by electroporation, <.45 microM "particles" were produced that could infect cells bearing the ecotropic viral receptor, causing these cells to replicate the vector RNAs and fuse with adjacent cells. We believe that the "particles" are fragments of cell membrane surrounding vector RNA. This system has promise as a novel vaccine agent and for study of the evolution of viruses.